It is known to provide protection for polyphase motors by placing a protector in heat conductive relationship with the windings of such motors using a snap acting thermostatic disc mounting electrical contacts which are adapted to move from a contacts closed position, engaged with respective stationary contacts in a normal circuit operational mode, to a contacts open position, disengaged from the stationary contacts upon the occurrence of selected elevated temperature conditions when the disc snaps from one dished configuration to an opposite dished configuration.
In U.S. Pat. No. 4,866,408, a protector is shown and described in which a pair of terminal pins extend through glass beads in a header plate. Each terminal pin mounts an end of a respective elongated strip heater that extends further away from the header plate and each strip heater mounts a respective stationary contact at the free end thereof. A rigid support member, attached to the header plate intermediate to the terminal pins, extends from the header plate and mounts a third heater which extends back toward but short of the header plate. A thermostatic snap acting disc is cantilever mounted to the free end of a third heater, the disc extending back over the third heater and mounting two movable electrical contacts adapted to move into and out of engagement with the stationary contacts. The switch is calibrated by adjustment of screws to vary the vertical position of the stationary contacts, as desired. Following calibration, a cover member is disposed over the switch mechanism and is hermetically attached to the header plate.
Although the above described switch is widely used, inexpensive and very effective, there is a need to provide a smaller switch and one which is reliable and even less expensive.
It is an object of the present invention to provide a protector particularly useful for polyphase motors, e.g., three phase motors, such as those used in scroll compressors, which is smaller than the prior art protector noted above yet which is reliable and inexpensive to make. Another object of the invention is the provision of a motor protector particularly useful for three phase motors having wye-connected windings which is smaller than conventional protectors yet one, despite having decreased thermal mass, has an optimum off time for ultimate trip conditions. Another object of the invention is the provision of a motor protector having a low profile with respect to the height of the protector relative to the header thereof without having the need for separate arc shielding ceramic pieces common in the prior art for protecting the glass mounting the terminal pins of the header.